Continuous fueling nuclear fission reactors differ from conventional nuclear fission reactors in that the fuel source (i.e., a fissionable material that may include any of the known fissionable isotopes, such as, but not limited to, U-235, U-233, or Pu-239, or may also contain fertile isotopes, such as, for example, U-238 or Th-232, that convert to fissile materials upon residence in an operating reactor core) is continuously provided to the reactor. In conventional or periodically fueled types, the reactor is initially provided with a fuel source or “load.” The reactor is then operated until the fuel load is depleted, at which point the reactor must be shut down and refueled. One type of continuous fueling reactor is the so-called pebble bed reactor, which takes its name from the pebble-like configuration of the fuel elements comprising the fuel source or load. The fuel source of a pebble bed reactor comprises a plurality of spherical elements or “pebbles,” each of which is about the size of a tennis ball. Each pebble is made up of a large number of much smaller coated fuel particles or kernels dispersed in a graphite matrix within the pebble.
A typical pebble bed reactor comprises a core formed by a plurality of the generally spherically shaped fuel elements or pebbles. The pebbles comprising the core are typically contained in a graphite reflector. A coolant, typically gaseous helium, is caused to flow through the pebble core and the graphite reflector. The pebble bed reactor is designed so that it is continuously replenished with fuel during operation. To date, three different types of continuous refueling systems have been used or proposed. In the first type of system, spent fuel pebbles are continuously extracted from the core and replaced with new fuel pebbles. The spent pebbles are usually extracted from the bottom of the core, whereas the new pebbles are provided to the top of the core. Thus, as the reactor is operated, fresh fuel pebbles located at the top of the core move steadily downward through the core as they are “burned,” ultimately being removed from the bottom of the core as spent fuel pebbles.
The second type of system is similar to the first, except the pebbles removed from the bottom of the core are recycled to the top of the core, whereupon they intermix with fresh fuel pebbles that are also provided to the top of the core. This type of system may also be provided with a depletion detection system for detecting when the pebbles extracted from the core have become depleted to the extent where it is no longer desirable to recycle them. The depleted pebbles are then removed from the recycle loop.
The third type of system differs from the first two in that no fuel is ever removed from it. In this case, as fuel is depleted, fresh fuel is added on top in order to maintain a critical core. The active portion of the core gradually moves upward until there is no longer room in the reactor core vat for additional fresh fuel.
Continuous fueling reactors, such as pebble bed reactors, are advantageous in that they provide a high-temperature heat supply with a high degree of fuel burn-up. The high-temperature heat supply provided by such pebble bed reactors may be useful in its own right and also allows for electricity generation at high thermal efficiencies. Pebble bed reactors are also considerably safer in operation compared with reactors that are periodically fueled. The construction of the fuel pebbles used in pebble bed reactors also present advantages in terms of unwanted proliferation of fissionable material.